Chemically-strengthened glass

ABSTRACT

The purpose of the present invention is to provide a chemically-strengthened glass exhibiting both surface strength and abrasion-resistant anti-fingerprint (AFP) properties. The present invention relates to a plate-shaped chemically-strengthened glass which has a compressive stress layer provided to a glass surface layer, a glass surface hydrogen concentration profile in a specific range, and a surface strength and abrasion-resistant anti-fingerprint (AFP) properties which are in specific ranges.

TECHNICAL FIELD

The present invention relates to a chemically strengthened glass.

BACKGROUND ART

In recent years, chemically strengthened glasses have been used as coverglasses of various display devices. It is requested to further improvethe strength thereof. Conventionally, it has been known that a glass isimmersed in a molten salt of potassium nitrate or the like to therebyperform a chemical strengthening treatment on the glass in order toenhance the surface strength of the glass. For example, PatentLiterature 1 discloses that a glass is immersed in a molten salt ofpotassium nitrate to thereby perform a chemical strengthening treatmenton the glass in order to enhance the surface strength of the glasssheet. In addition, Patent Literature 1 discloses that the glass sheetsubjected to the chemical strengthening treatment is then subjected to asurface etching treatment, in order to further improve the surfacestrength of the glass sheet subjected to the chemical strengtheningtreatment.

On the other hand, in order to attain reduction in weight and reductionin thickness, a so-called OGS (One Glass Solution) type display devicehas been developed (Patent Literature 2). In the OGS type displaydevice, a chemically strengthened glass is equipped directly with touchsensors to omit a glass sheet, and the chemically strengthened glassequipped with the touch sensors is disposed on a liquid crystal display(LCD).

In order to impart an antifouling property, a scratch resistanceproperty, a surface slip property and a function of reducing adhesion offingerprints, a surface of a chemically strengthened glass may be coatedwith an AFP (Anti Finger Print) agent including a silicone-basedcompound, a fluorine-based compound or a composition containing thosecompounds by a method such as deposition, spraying or dipping, so as toform an AFP film (Patent Literature 3). An excellent abrasion-resistantproperty of the AFP film is required so that the desired antifoulingproperty, scratch resistance property, surface slip property andfunction of reducing adhesion of fingerprints can be sustained for along time.

CITATION LIST Patent Literature

Patent Literature 1: JP-A-2013-516387

Patent Literature 2: JP-A-2011-197708

Patent Literature 3: JP-A-2000-144097

SUMMARY OF INVENTION Technical Problem

However, in the chemically strengthened glass obtained by themanufacturing method described in Patent Literature 1, there is aproblem that abrasion-resistant property of the AFP film formed with theAFP agent applied to the glass surface (hereinafter also referred to as“AFP abrasion resistance”) is so low that the surface strength is notsufficient.

Therefore, in consideration of the aforementioned circumstances, anobject of the present invention is to provide an unconventionalchemically strengthened glass having both surface strength and AFPabrasion resistance.

Solution to Problem

The present inventors have made extensive and intensive investigations.As a result, it has been found that surface strength and AFP abrasionresistance can be attained compatibly in a chemically strengthened glassin which a hydrogen concentration profile in a surface layer is within aspecific range, hereby leading to accomplishment of the presentinvention.

That is, the present invention relates to the following configurations.

-   1. A chemically strengthened glass having a sheet shape and    including a compressive stress layer in a glass surface layer, in    which:

a straight line linearly approximating a profile of a hydrogenconcentration Y with respect to a depth X from an outermost surface of aglass sheet at X=0.1 to 0.6 (μm) satisfies Expression (I);

a surface strength F (N) measured on conditions below by a ball-on-ringstrength test satisfies a relation of F≥1500×t² with respect to a sheetthickness t (mm) of the glass sheet; and

a contact angle is 65° or more after 4,000 times of an abrasion when anAFP abrasion resistance is measured on conditions below by a rubbereraser abrasion test:

Y=aX+b  (I),

in which:

-   -   Y is the hydrogen concentration (mol/L, measured as H₂O);    -   X is the depth from the outermost surface of the glass sheet        (μm);    -   a is −0.450 to −0.300; and    -   b is 0.250 to 0.400,

Ball-on-Ring (BoR) Strength Test Conditions:

a glass sheet having a sheet thickness t (mm) is disposed on a stainlesssteel ring having a diameter of 30 mm and including a contact portionrounded with a curvature radius of 2.5 mm, and a steel ball having adiameter of 10 mm is brought into contact with the glass sheet; in thisstate, the ball is loaded at a center of the ring under static loadconditions; a breaking load (in units of N) at which the glass is brokenis regarded as a BOR strength, and an average value of 20 timesmeasurements of the BOR strength is regarded as a surface strength F;here, a case where a breaking start point of the glass is 2 mm or moreaway from a loading point of the ball is excluded from data forcalculating the average value,

Rubber Eraser Abrasion Test Conditions:

a chemically strengthened glass sheet surface is cleaned by ultravioletrays, and sprayed and coated with OPTOOL (registered trademark) DSX(made by Daikin Industries, Ltd.) so as to form an AFP filmsubstantially uniformly on the glass sheet surface;

a rubber eraser (MINOAN made by Mirae Science Co., Ltd.) is attached toan indenter with 1 cm² wide; in a state where a load of 1 kgf is appliedto the rubber eraser, an AFP film surface formed on the glass sheetsurface is rubbed with the rubber eraser reciprocatively 4,000 timeswith a stroke width of 20 mm and at a velocity of 30 mm/sec; then, theAFP film surface is dry-wiped and cleaned with a cloth (DUSPER(registered trademark) made by Ozu Corporation); thereafter, watercontact angles (°) are measured at three places on the AFP film surface;this operation is repeated three times, and an average water contactangle (°) is measured from a total of nine water contact angles; thewater contact angles (°) on the AFP film surface are measured by amethod according to JIS R 3257 (1999).

-   2. The chemically strengthened glass according to the aforementioned    configuration 1, having a surface roughness (Ra) of 0.30 nm or more.-   3. The chemically strengthened glass according to the aforementioned    configuration 1 or 2, having a surface compressive stress value (CS)    of 600 MPa or more.-   4. The chemically strengthened glass according to any one of the    aforementioned configurations 1 through 3, having a depth of the    compressive stress layer (DOL) of 10 μm or more.-   5. The chemically strengthened glass according to any one of the    aforementioned configurations 1 through 4, having an internal    tensile stress (CT) of 72 MPa or less.-   6. The chemically strengthened glass according to any one of the    aforementioned configurations 1 through 5, which is used as a cover    glass of a display device.

Advantageous Effects of Invention

According to the present invention, it is possible to provide achemically strengthened glass having both surface strength and AFPabrasion resistance owing to a profile of hydrogen concentration in aglass sheet surface layer set within a specific range.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view illustrating a method of a ball-on-ringstrength test.

FIG. 2 is a schematic view showing steps of manufacturing a chemicallystrengthened glass according to the present invention.

FIG. 3 is a schematic view of an experimental system for making anatmosphere in a chemical strengthening treatment.

FIG. 4A and FIG. 4B are graphs showing results of AFP abrasionresistance evaluated by a rubber eraser abrasion test.

DESCRIPTION OF EMBODIMENTS

The present invention will be described below in detail. However, thepresent invention is not limited to the following embodiments, and maybe modified desirably to carry out without departing from the gistthereof. In addition, the word “to” designating a numerical range in thepresent description is used as a denotation of a range includingnumerical values on both sides of the word “to” as a lower limit valueand an upper limit value of the range. In addition, in the presentdescription, “mass %” and “mass ppm” are synonyms for “weight %” and“weight ppm” respectively. In addition, “ppm” written simply means“weight ppm”.

<Chemically Strengthened Glass>

The chemically strengthened glass of the present invention is achemically strengthened glass including a compressive stress layer in aglass sheet surface layer formed by ion exchange, in which:

a straight line linearly approximating a profile of a hydrogenconcentration Y with respect to a depth X from an outermost surface of aglass sheet at X=0.1 to 0.6 (μm) satisfies Expression (I);

a surface strength F (N) measured on conditions below by a ball-on-ringstrength test satisfies a relation of F≥1500×t² with respect to a sheetthickness t (mm) of the glass sheet; and

a contact angle is 65° or more after 4,000 times of an abrasion when anAFP abrasion resistance is measured on conditions below by a rubbereraser abrasion test.

Y=aX+b  (I),

in which:

-   -   Y is the hydrogen concentration (mol/L, measured as H₂O);    -   X is the depth from the outermost surface of the glass sheet        (μm);    -   a is −0.450 to −0.300; and    -   b is 0.250 to 0.400.

Ball-on-Ring (BoR) Strength Test Conditions:

A glass sheet having a sheet thickness t (mm) is disposed on a stainlesssteel ring having a diameter of 30 mm and including a contact portionrounded with a curvature radius of 2.5 mm, and a steel ball having adiameter of 10 mm is brought into contact with the glass sheet. In thisstate, the ball is loaded at a center of the ring under static loadconditions. A breaking load (in units of N) at which the glass is brokenis regarded as a BOR strength, and an average value of 20 timesmeasurements of the BOR strength is regarded as a surface strength F.Here, a case where a breaking start point of the glass is 2 mm or moreaway from a loading point of the ball is excluded from data forcalculating the average value.

Rubber Eraser Abrasion Test Conditions:

A chemically strengthened glass sheet surface is cleaned by ultravioletrays, and sprayed and coated with OPTOOL (registered trademark) DSX(made by Daikin Industries, Ltd.) so as to form an AFP filmsubstantially uniformly on the glass sheet surface.

A rubber eraser (MINOAN made by Mirae Science Co., Ltd.) is attached toan indenter with 1 cm² wide. In a state where a load of 1 kgf is appliedto the rubber eraser, an AFP film surface formed on the glass sheetsurface is rubbed with the rubber eraser reciprocatively 4,000 timeswith a stroke width of 20 mm and at a velocity of 30 mm/sec. Then, theAFP film surface is dry-wiped and cleaned with a cloth (DUSPER(registered trademark) made by Ozu Corporation). Thereafter, watercontact angles (°) are measured at three places on the AFP film surface.This operation is repeated three times, and an average water contactangle (°) is measured from a total of nine water contact angles. Thewater contact angles (°) on the AFP film surface are measured by amethod according to JIS R 3257 (1999).

The chemically strengthened glass according to the present invention hasan ion-exchanged compressive stress layer in the glass sheet surfacelayer. In the present description, the compressive stress layer is ahigh density layer formed by bringing the glass sheet into contact withinorganic salt such as potassium nitrate to cause ion exchange betweenmetal ions (Na ions) in the glass sheet surface and ions (K ions) havinglarge ionic radii in the inorganic salt. Owing to the increase ofdensity in the glass sheet surface, a compressive stress is generated sothat the glass sheet can be strengthened.

(Glass Composition)

It will go well if a glass for use in the present invention containsalkali ions. Glasses having various compositions can be used as long asthey have components capable of being formed and strengthened by achemical strengthening treatment. Among them, the glass preferablycontains sodium. Specific examples of such glasses includealuminosilicate glass, soda lime glass, borosilicate glass, lead glass,alkali barium glass, and aluminoborosilicate glass.

Although the composition of a glass for use as the chemicallystrengthened glass according to the present invention is notparticularly limited, the following glass compositions may be used, forexample.

-   (1) A glass containing 50 to 74% of SiO₂, 1 to 15% of Al₂O₃, 6 to    18% of Na₂O, 0 to 3% of K₂O, 2 to 15% of MgO, 0 to 6% of CaO, 0-5%    of ZrO₂, and 0 to 1% of TiO₂ as a composition expressed by mol % on    an oxide basis, where a total content of SiO₂ and Al₂O₃ is 76% or    less, and a total content of Na₂O and K₂O is 12 to 25%.-   (2) A glass containing 55.5 to 80% of SiO₂, 8 to 20% of Al₂O₃, 8 to    25% of Na₂O, 0 to 3% of K₂O, 0 to 1% of TiO₂, 0-5% of ZrO₂, and 1%    or more of alkaline earth metal RO (RO designates MgO+CaO+SrO+BaO)    as a composition expressed by mol % on an oxide basis.-   (3) A glass containing 56 to 72% of SiO₂, 8 to 20% of Al₂O₃, 3 to    20% of B₂O₃, 8 to 25% of Na₂O, 0 to 5% of K₂O, 0 to 15% of MgO, 0 to    15% of CaO, and 0 to 1% of TiO₂ as a composition expressed by mol %    on an oxide basis.-   (4) A glass containing 60 to 72% of SiO₂, 1 to 18% of Al₂O₃, 1 to 5%    of MgO, 0 to 5% of CaO, 12 to 19% of Na₂O, and 0 to 5% of K₂O, and    containing 1% or more of alkaline earth metal RO (RO designates    MgO+CaO+SrO+BaO), as a composition expressed by mass % on an oxide    basis.-   (5) A glass containing 55.5 to 80% of SiO₂, 12 to 20% of Al₂O₃, 8 to    25% of Na₂O, 2.5% or more of P₂O₅, and 1% or more of alkaline earth    metal RO (RO designates MgO+CaO+SrO+BaO) as a composition expressed    by mol % on an oxide basis.-   (6) A glass containing 57 to 76.5% of SiO₂, 12 to 18% of Al₂O₃, 8 to    25% of Na₂O, 2.5 to 10% of P₂O₅, and 1% or more of alkaline earth    metal RO as a composition expressed by mol % on an oxide basis.-   (7) A glass containing 56 to 72% of SiO₂, 8 to 20% of Al₂O₃, 3 to    20% of B₂O₃, 8 to 25% of Na₂O, 0 to 5% of K₂O, 0 to 15% of MgO, 0 to    15% of CaO, 0-15% of SrO₂, 0 to 15% of BaO, and 0 to 8% of ZrO₂ as a    composition expressed by mol % on an oxide basis.

(Hydrogen Concentration)

In the chemically strengthened glass according to the present invention,a profile of a hydrogen concentration in a glass sheet surface layer iswithin a specific range. Specifically, a straight line obtained bylinearly approximating a profile of a hydrogen concentration Y withrespect to a depth X from an outermost surface of the glass sheet atX=0.1 to 0.6 (μm) satisfies the following Expression (I).

Y=aX+b  (I)

[signs in Expression (I) have the following meanings respectively:

Y: hydrogen concentration (mol/L, measured as H₂O);

X: depth from the outermost surface of the glass sheet (μm);

a is −0.450 to −0.300; and

b is 0.250 to 0.400]

As for the strength of a glass sheet, it has been known that thestrength of the glass sheet is lowered by the existence of hydrogen(moisture) in the glass. The present inventors found that the strengthof a glass may be lowered after a chemical strengthening treatment, andthe major cause of that is due to the fact that moisture in theatmosphere intrudes the glass to cause chemical defects.

When a glass has a high hydrogen concentration, hydrogen enters anSi—O—Si bond network of the glass to form Si—OH bond. Thus, the Si—O—Sibond is disconnected. As the hydrogen concentration in the glass ishigher, it can be considered that parts where the Si—O—Si bond isdisconnected are increased, and thus chemical defects tend to begenerated to lower the strength.

The aforementioned Expression (I) is established in the region of thedepth X=0.1 to 0.6 (μm) from the outermost surface of the glass sheet.The thickness of a compressive stress layer formed by ion exchangedepends on the degree of chemical strengthening, but it is normallyformed within a range of from 5 to 90 μm. The intrusion depth ofhydrogen into the glass depends on a diffusion coefficient, temperatureand time, and the intrusion amount of hydrogen depends on the moistureamount in the atmosphere in addition to those factors. The hydrogenconcentration after the chemical strengthening is the highest in theoutermost surface and decreases gradually toward a deeper portion (bulk)where the compressive stress layer is not formed. The decreasing degreeof the hydrogen concentration is prescribed in the aforementionedExpression (I). In the outermost surface (X=0 μm), the moistureconcentration is likely to vary due to deterioration with time.Therefore, it is assumed that Expression (I) is established in anear-surface region (X=0.1 to 0.6 (μm)) which is assumed not to beaffected by the variation of the moisture concentration in the outermostsurface.

In Expression (I), a designates an inclination prescribing thedecreasing degree of the hydrogen concentration. The range of a is−0.450 to −0.300, preferably −0.380 to −0.300, and more preferably−0.350 to −0.300. On the other hand, in Expression (I), b corresponds tothe hydrogen concentration at the outermost surface (X=0 μm). The rangeof b is 0.250 to 0.400, preferably 0.250 to 0.370, and more preferably0.250 to 0.320.

Generally it is considered that a very small crack existing in a surfaceof a glass sheet is extended by external mechanical pressure, therebycausing reduction in strength of the glass sheet. As the glass structureat the tip of the crack is richer in Si—OH, the crack is extended moreeasily (Won-Taek Han et al., “Effect of residual water in silica glasson static fatigue”, Journal of Non-Crystalline Solids, 127, (1991)97-104). On the assumption that the tip of the crack is exposed to theatmosphere, it is estimated that the Si—OH content at the tip of thecrack indicates a positive correlation with the hydrogen concentrationin the outermost surface of the glass. Accordingly, it is preferablethat b corresponding to the hydrogen concentration in the outermostsurface is preferably within the aforementioned range.

The intrusion depth of hydrogen is highly likely to vary depending onthe conditions of the chemical strengthening treatment. However, on theassumption that the intrusion depth of hydrogen does not vary, anegative correlation appears between b corresponding to the hydrogenconcentration in the outermost surface and a corresponding to theinclination prescribing the decreasing degree of the hydrogenconcentration. It is therefore preferable that a is preferably withinthe aforementioned range.

(Method for Measuring Hydrogen Concentration Profile)

Here, the hydrogen concentration profile (H₂O concentration, mol/L) ofthe glass sheet is a profile measured under the following analysisconditions.

Secondary ion mass spectrometry (SIMS) is used for measuring thehydrogen concentration profile of the glass sheet. To obtain aquantitative hydrogen concentration profile by the SIMS, a standardspecimen whose hydrogen concentration has been known is required. Amethod for preparing a standard specimen and a method for quantitativelydetermining a hydrogen concentration will be described below.

-   1) A part of a glass sheet to be measured is cut out.-   2) A region at least 50 μm deep from a surface of the glass sheet    thus cut out is removed by polishing or chemical etching. Removing    treatment is performed on both the opposite sides of the glass    sheet. That is, the removed thickness is at least 100 μm in total on    the opposite sides. The glass sheet subjected to the removing    treatment is regarded as a standard specimen.-   3) Infrared spectroscopy (IR) is applied to the standard specimen to    obtain absorbance height A₃₅₅₀ of a peak top near 3,550 cm⁻¹ and    absorbance height A₄₀₀₀ at 4,000 ⁻¹ (base line) in an IR spectrum.-   4) A sheet thickness d (cm) of the standard specimen is measured    using a sheet thickness meter such as a micrometer.-   5) With reference to Literature A, the hydrogen concentration    (mol/L, measured as H₂O) of the standard specimen is obtained using    the following Expression (II) on the assumption that the infrared    practical absorbance index ε_(pract) (L/(mol·cm)) of H₂O in the    glass is 75.

Hydrogen concentration of standard specimen=(A ₃₅₅₀ −A ₄₀₀₀)/(ε_(pract)·d)  Expression (II)

Literature A) S. Ilievski et al., Glastech. Ber. Glass Sci. Technol., 73(2000) 39.

The glass sheet to be measured and the standard specimen with the knownhydrogen concentration obtained by the aforementioned method areconveyed into an SIMS apparatus simultaneously, and measuredsequentially to obtain depth-direction profiles of ¹H⁻ and ³⁰Si⁻intensities. After that, the ¹H⁻ profile is divided by the ³⁰Si⁻ profileto obtain a depth-direction profile of the ¹H⁻/³⁰Si⁻ intensity ratio.From the depth-direction profile of the ¹H⁻/³⁰Si⁻ intensity ratio in thestandard specimen, an average ¹H⁻/³⁰Si⁻ intensity ratio in a depthregion of from 0.1 μm to 0.6 μm is calculated, and a calibration curvebetween the value of the average ¹H⁻/³⁰Si⁻ intensity ratio and thehydrogen concentration is created to pass through an origin (calibrationcurve on single standard specimen). By use of the calibration curve, the¹H⁻/³⁰Si⁻ intensity ratio in the ordinate of the profile of the glasssheet to be measured is converted into hydrogen concentration. Thus, ahydrogen concentration profile of the glass sheet to be measured isobtained. Incidentally, SIMS and IR measuring conditions will bedescribed below.

(Measuring Conditions of SIMS)

Apparatus: ADEPT1010 made by ULVAC-PHI, Incorporated

Primary Ion Species: Cs⁺

Primary Ion Acceleration Voltage: 5 kV

Primary Ion Current Value: 500 nA

Primary Ion Incident Angle: 60° with respect to normal line of specimensurface

Primary Ion Raster Size: 300×300 μm²

Secondary Ion Polarity: minus

Secondary Ion Detection Region: 60×60 μm² (4% of primary ion rastersize)

ESA Input Lens: 0

Use of Neutralization Gun: yes

Method for Converting Abscissa from Sputtering Time to Depth: Depth ofan analysis crater is measured by a stylus type surface shape measuringinstrument (Dektak 150 made by Veeco Instruments Inc.) to obtain aprimary ion sputtering rate. By use of this sputtering rate, theabscissa is converted from the sputtering time to the depth.

Field Axis Potential upon ¹H⁻ detection: An optimum value may varydepending on the apparatus. A measurer sets the value carefully to cutthe background sufficiently.

(Measuring Conditions of IR)

Apparatus: Nic-plan/Nicolet 6700 made by Thermo Fisher Scientific Inc.

Resolution: 8 cm⁻¹

Integration: 64

Detector: TGS detector

From the hydrogen concentration profile (H₂O concentration, mol/L) ofthe glass sheet measured under the aforementioned analysis conditions,Expression (I) is derived in the following procedure. Linearapproximation is performed on the profile of the hydrogen concentrationY in the region of the depth X=0.1 to 0.6 (μm) from the outermostsurface of the glass sheet. An expression of an approximate straightline obtained thus is set as Expression (I).

Examples of means for controlling a and b include changing theconcentration of flux, the concentration of sodium, the temperature, thetime, etc. in the chemical strengthening treatment.

(Strength of Glass Sheet)

The strength (surface strength) of the chemically strengthened glasssheet according to the present invention can be evaluated by a BoR(Ball-on-Ring) strength test. Specifically the BoR strength test isperformed as follows.

The chemically strengthened glass according to the present invention isevaluated by BoR strength F (N) measured by a BoR strength testperformed as follows. That is, a glass sheet with a sheet thickness t(mm) is placed on a ring-like reception jig having a diameter of 30 mm.The ring-like reception jig is made of stainless steel and includes acontact portion rounded with a curvature radius of 2.5 mm. In a statewhere a pressure jig 2 (made of quenched steel, sphere having a diameterof 10 mm and mirror-finished) made of SUS304 is brought into contactwith the glass sheet, the pressure jig 2 is loaded at the center of thering-like reception jig under static load conditions.

FIG. 1 shows a schematic view for explaining the BoR strength test usedin the present invention. In the BoR strength test, a glass sheet 1which has been mounted horizontally is pressurized by use of thepressure jig 2 (made of quenched steel, having a diameter of 10 mm, andmirror-finished) which is a ball made of SUS304. Thus, the strength ofthe glass sheet 1 is measured.

In FIG. 1, the glass sheet 1 which serves as a sample is placedhorizontally on the reception jig 3 (having a diameter of 30 mm,including a contact portion having a curvature radius R of 2.5 mm andmade of quenched steel, and mirror-finished) made of SUS304. Thepressure jig 2 for pressurizing the glass sheet 1 is placed above theglass sheet 1.

In the present embodiment, the central region of the glass sheet 1obtained in each of Examples and Comparative Examples is pressurizedfrom above the glass sheet 1. Incidentally, test conditions will bedescribed below.

Descending Rate of Pressure Jig 2: 1.0 (mm/min)

On this occasion, a breaking load (in units of N) with which the glasssheet is broken is regarded as BoR strength, and an average value of BoRstrength measured 20 times is regarded as BoR average strength. When abreaking start point of the glass sheet is 2 mm or more away from theposition where the ball is pressed, the measured BoR strength isexcluded from data for calculating the average value.

The chemically strengthened glass according to the present inventionsatisfies the relation of F≥1500×t², more preferably the relation ofF≥1800×t², even more preferably the relation of F≥2000×t², andparticularly preferably the relation of F≥2100×t². (In the expression, Fdesignates BoR strength (N) measured by the BoR strength test, and tdesignates the sheet thickness (mm) of the glass sheet.). When the BoRstrength F (N) is within such a range, the chemically strengthened glasscan exhibit excellent strength even if it is formed into a thin sheet.

(AFP Abrasion Resistance)

The AFP abrasion resistance in the chemically strengthened glassaccording to the present invention can be evaluated by the followingrubber eraser abrasion test.

(Rubber Eraser Abrasion Test) Test Conditions:

A surface of a chemically strengthened glass sheet is cleaned byultraviolet rays, and sprayed and coated with OPTOOL (registeredtrademark) DSX (made by Daikin Industries, Ltd.) so as to form an AFPfilm substantially uniformly on the glass sheet surface.

A rubber eraser (MINOAN made by Mirae Science Co., Ltd.) is attached toan indenter with 1 cm² wide. In a state where a load of 1 kgf is appliedto the rubber eraser, the surface of the AFP film formed on the glasssheet surface is rubbed with the rubber eraser reciprocatively 4,000times with a stroke width of 20 mm and at a velocity of 30 mm/sec. Afterthat, the AFP film surface is dry-wiped and cleaned with a cloth (DUSPER(registered trademark) made by Ozu Corporation). Then, water contactangles (°) are measured at three places on the AFP film surface. Thisoperation is repeated three times, and an average water contact angle(°) is measured from the total of nine water contact angles.

The water contact angles (°) on the AFP film surface are measured by amethod according to JIS R 3257 (1999).

The aforementioned rubber eraser abrasion test can be performed by useof a commercially available tester (for example, plane abrasion tester(triple type) model: PA-300A made by Daiei Kagaku Seiki Mfg. Co., Ltd.)or the like.

In the chemically strengthened glass according to the present invention,the AFP abrasion resistance measured under the aforementioned conditionsby the rubber eraser abrasion test is 65° or more, preferably 70° ormore, more preferably 75° or more, and particularly preferably 80° ormore in contact angle after 4,000 times of rubbing. When the contactangle is 65° or more, an excellent antifouling property, an excellentscratch resistance property and an excellent surface slip property canbe sustained for a long time. It is more preferable that the contactangle is larger. However, the contact angle may be 125° or less as anupper limit value thereof.

The chemically strengthened glass according to the present invention canexhibit excellent AFP abrasion resistance in various situations when theAFP abrasion resistance measured under the aforementioned conditions bythe aforementioned rubber eraser abrasion test is within theaforementioned range.

(Surface Roughness)

In the chemically strengthened glass according to the present invention,surface roughness (Ra) is preferably 0.30 nm or more, more preferably0.40 nm or more, and even more preferably 0.50 nm or more in order toenhance the AFP abrasion resistance. When the surface roughness is 0.30nm or more, the AFP abrasion resistance can be enhanced. It is morepreferable that the surface roughness (Ra) is higher. However, thesurface roughness may be 2.0 nm or less, preferably 1.5 nm or less, andmore preferably 1.0 nm or less as an upper limit value thereof. When thesurface roughness (Ra) is made 2.0 nm or less, it is possible to preventwhite cloudiness of the glass to thereby suppress deterioration inquality of its appearance. The surface roughness is measured in a 1μm×0.5 μm measurement region by AFM (Atomic Force Microscope) surfaceobservation.

(Depth of Compressive Stress Layer, and Surface Compressive StressValue)

The compressive stress layer is a high density layer formed by ionexchange between Na ions in the glass surface and K ions in a moltensalt when the glass is brought into contact with the inorganic salt suchas potassium nitrate.

The depth of the compressive stress layer (DOL) can be measured by useof an EPMA (Electron Probe Micro Analyzer), a surface stress meter (e.g.FSM-6000 made by Orihara Industrial Co., Ltd.) or the like.

In the chemically strengthened glass according to the present invention,the depth of the compressive stress layer is preferably 10 μm or more,more preferably 15 μm or more, even more preferably 20 μm or more, andparticularly preferably 25 μm or more in order to provide sufficientstrength to the glass. On the other hand, the upper limit of the depthof the compressive stress layer is not particularly limited, but istypically 140 μm or less.

The thickness of a low-density layer to be removed by an acid treatmentstep or an alkali treatment step which will be described later is fromabout 10 nm to about 1,000 nm at the most. Accordingly, as for the depthof the compressive stress layer in the chemically strengthened glassaccording to the present invention, the depth of the compressive stresslayer formed in the chemical strengthening treatment is substantiallyequal to the depth of the compressive stress layer which has beensubjected to the acid treatment step or the alkali treatment step.

The surface compressive stress value (CS) of the chemically strengthenedglass according to the present invention is preferably 600 MPa or more,more preferably 650 MPa or more, even more preferably 700 MPa or more,and particularly preferably 800 MPa or more. On the other hand, althoughthe upper limit is not particularly limited, the surface compressivestress value is typically 1,400 MPa or less.

The surface compressive stress value can be measured by use of an EPMA(Electron Probe Micro Analyzer), a surface stress meter (e.g. FSM-6000made by Orihara Industrial Co., Ltd.) or the like. The surfacecompressive stress value can be calculated by use of a stress profilecalculation method disclosed in JP-A-2016-142600.

The internal tensile stress (CT) of the chemically strengthened glassaccording to the present invention is preferably 72 MPa or less, morepreferably 62 MPa or less, and even more preferably 52 MPa or less. Onthe other hand, although the lower limit is not particularly limited,the internal tensile stress is typically 20 MPa or more. A CT value isobtained in such a manner that a stress distribution is measured, andthe obtained stress distribution is integrated by thickness.

<Method for Manufacturing Chemically Strengthened Glass>

As a method for manufacturing a chemically strengthened glass accordingto the present invention, a method including the following steps (a) to(d) can be exemplified.

-   (a) a step of preparing a glass sheet containing alkali ions;-   (b) a step of preparing inorganic salt containing other alkali ions    larger in ionic radii than the alkali ions contained in the glass    sheet;-   (c) a step of performing ion exchange between the alkali ions in the    glass sheet and the other alkali ions in the inorganic salt in an    atmosphere with a dew point of 20° C. or higher; and-   (d) a step of removing a part of the ion-exchanged surface of the    glass sheet

Each of the steps will be described below.

((a) Step of Preparing a Glass Sheet Containing Alkali Ions)

Although a method for manufacturing a glass is not particularly limited,the glass can be manufactured as follows. A desired glass raw materialsare put into a continuous melting furnace. The glass raw materials areheated and melted preferably at 1,500 to 1,600° C., and clarified. Afterthat, the molten glass is supplied to a forming apparatus and formedinto a sheet-like shape, then followed by being gradually cooled.

Incidentally, various methods can be used for forming the glass sheet.Examples of the forming methods include a down draw process (such as anoverflow down draw process, a slot down process, a redraw process,etc.), a float process, a roll-out process, a press process, etc. Amongthose processes, the float process is preferred. Because in the floatprocess, at least a part of a glass sheet surface may be cracked, andtherefore the effect of the present invention can be confirmedconspicuously.

The sheet thickness of the glass sheet is not particularly limited.However, in order to perform the chemical strengthening treatmenteffectively, the thickness is normally preferably 5 mm or less, morepreferably 3 mm or less, even more preferably 1 mm or less, andparticularly preferably 0.7 mm or less.

In addition, the shape of the glass sheet used in the present inventionis not particularly limited. For example, glass sheets having variousshapes such as a flat sheet shape with a uniform sheet thickness, ashape with a curved surface in at least one of the front surface and theback surface, a cubic shape with a bent portion or the like, etc. can beused.

((b) Step of Preparing Inorganic Salt Containing Other Alkali IonsLarger in Ionic Radii Than the Alkali Ions Contained in the Glass Sheet)

The chemically strengthened glass according to the present invention hasan ion-exchanged compressive stress layer in the glass sheet surfacelayer. In an ion exchange process, the surface of the glass sheet ision-exchanged to form a surface layer where a compressive stressremains. Specifically, alkali metal ions (Li ions and/or Na ions) withsmall ionic radii in the glass sheet surface are substituted with otheralkali ions (Na ions and/or K ions) with larger ionic radii by ionexchange at a temperature equal to or lower than a glass transitionpoint. Thus, the compressive stress remains on the surface of the glasssheet, and the strength of the glass sheet is improved.

As the method for manufacturing a chemically strengthened glassaccording to the present invention, a chemical strengthening treatmentis carried out by ion exchange in which a glass containing alkali ionsas described previously is brought into contact with inorganic saltcontaining other alkali ions with larger ionic radii than the alkaliions contained in the glass. That is, the alkali ions contained in theglass are ion-exchanged with the other alkali ions contained in theinorganic salt.

When the alkali ions contained in the glass are Na ions, it ispreferable that the inorganic salt contains potassium nitrate (KNO₃). Itis more preferable that the inorganic salt further contains at least onekind of salt selected from the group consisting of K₂CO₃, Na₂CO₃, KHCO₃,NaHCO₃, Li₂CO₃, Rb₂CO₃, Cs₂CO₃, MgCO₃, CaCO₃, and BaCO₃.

For example, when the inorganic salt contains potassium nitrate, thepotassium nitrate has a melting point at 330° C., which is not higherthan the strain point (normally 500 to 600° C.) of the glass to bechemically strengthened. In addition, the aforementioned salts excludingthe potassium nitrate (hereinafter also referred to as “flux”) has aproperty of disconnecting a network of the glass represented by Si—O—Sibond. The temperature at which the chemical strengthening treatment isperformed is several hundred ° C., which is so high that Si—O covalentbonds of the glass are cut suitably at the temperature. Thus, a densityreduction treatment which will be described later tends to proceed.

Incidentally, the degree of cutting of the covalent bonds differsdepending on the conditions of the chemical strengthening treatment,such as the glass composition, the kind of the salt (flux), thetemperature or the time for the chemical strengthening treatment, etc.However, it is considered that it is preferable to select conditionswith which, of four covalent bonds extending from Si, one or two bondscan be cut.

A high density compressive stress layer is formed by ion exchangebetween Na ions (or Li ions) in the glass sheet surface and K ions (orNa ions) in the inorganic salt. As a method for bringing the glass sheetinto contact with the inorganic salt, a method for applying thepaste-like inorganic salt to the glass, a method for spraying an aqueoussolution of the inorganic salt to the glass, a method for immersing theglass into a salt bath of the molten salt heated to the melting pointthereof or higher, etc. can be used. Among them, the method forimmersing the glass into the molten salt is preferred.

The inorganic salt may contain, in addition to the potassium nitrate andthe flux, other chemical species as long as they do not impede theeffect of the present invention. Examples of the other chemical speciesinclude alkali chloride or alkali borate such as sodium chloride,potassium chloride, sodium borate, potassium borate. One of those may beadded alone, or some kinds of them may be added in combination.

In the molten salt used in the method for manufacturing the chemicallystrengthened glass according to the present invention, the Naconcentration is preferably 500 weight ppm or more, and more preferably1,000 weight ppm or more. It is further more preferable that the Naconcentration in the molten salt is 2,000 weight ppm or more because thelow density layer can be made deeper by the acid treatment step whichwill be described later. The upper limit of the Na concentration is notparticularly limited, but it is allowed to be high enough to obtain adesired surface compressive stress (CS).

((c) Step of Performing Ion Exchange Between the Alkali Ions in theGlass Sheet and the Other Alkali Ions in the Inorganic Salt in anAtmosphere with a Dew Point of 20° C. or Higher)

The step (c) is a step in which the glass sheet prepared in the step (a)is subjected to an ion exchange treatment (chemical strengtheningtreatment) using the molten salt prepared in the step (b). The chemicalstrengthening treatment is carried out by immersing the glass sheet intothe molten salt to thereby achieve ion exchange (substitution) betweenthe alkali ions (Li ions or Na ions) in the glass and the other alkaliions (Na ions or K ions) with larger ionic radii in the molten salt.Owing to the ion exchange, the composition of the glass sheet surfacecan be changed to form a compressive stress layer 20 in which thedensity in the glass sheet surface has increased ((a) and (b) of FIG.2). By the increase of the density in the glass sheet surface, acompressive stress is generated so that the glass sheet can bestrengthened.

Incidentally, in fact, the density of the chemically strengthened glassis gradually increased from an outer edge of an intermediate layer 30(bulk) located at the center of the glass sheet toward the surface ofthe compressive stress layer. Therefore, between the intermediate layer30 and the compressive stress layer 20, there is no clear boundary wherethe density varies suddenly. Here, the intermediate layer designates alayer which is located in a central portion of the glass sheet andbetween compressive stress layers on the opposite sides. Differentlyfrom the compressive stress layer, the intermediate layer is a layerwhich has not been subjected to the ion exchange.

Specifically the chemical strengthening treatment (ion exchangetreatment) can be performed in the following procedure. First, the glasssheet is preheated, and the aforementioned molten salt is adjusted to atemperature for chemical strengthening. Next, the preheated glass sheetis immersed into the molten salt in a molten salt bath for apredetermined time. The glass sheet is then lifted up from the moltensalt, and cooled. Incidentally, it is preferable that the glass sheet issubjected to profiling, for example, mechanical processing such ascutting, end face processing and perforating, in accordance with itsusage before the chemical strengthening treatment.

It is preferable that the temperature of the chemical strengthening isnot higher than the strain point (normally 500 to 600° C.) of the glassto be strengthened. Particularly in order to obtain a larger depth ofthe compressive stress layer, the temperature is preferably 350° C. orhigher. In order to shorten the treatment time and accelerate theformation of a low density layer, the temperature is more preferably400° C. or higher, and further more preferably 430° C. or higher.

In the method for manufacturing the chemically strengthened glassaccording to the present invention, the water vapor content in themolten salt when the glass sheet is dipped therein is increased, so thata low density layer formed in a step in which the glass sheet is broughtinto contact with acid as will be described later can be thickened. In astep in which the glass sheet is brought into contact with alkali, thelow density layer can be removed. Therefore, when the thickness of thelow density layer is made equal to or larger than the average depth ofcracks or latent scratches existing in the glass sheet surface, thecracks or the latent scratches can be removed together with the removalof the low density layer. Thus, the excellent surface strength can beachieved in the chemically strengthened glass.

The step of performing the ion exchange is preferably performed in anatmosphere whose dew point temperature is 20° C. or higher. The dewpoint temperature is more preferably 30° C. or higher, even morepreferably 40° C. or higher, particularly preferably 50° C. or higher,and most preferably 60° C. or higher. In addition, it is preferable thatthe upper limit of the dew point temperature is made not higher than thetemperature of the inorganic salt (molten salt) with which the ionexchange is carried out.

As for the dew point temperature (hereinafter also referred to as “dewpoint” simply), the dew point temperature at least in the vicinity ofthe boundary surface of the molten salt may be within the aforementionedrange. The vicinity of the boundary surface means an atmosphere in aregion of 200 mm or less away from the boundary surface of the moltensalt. The dew point can be measured by Vaisala DRYCAP (registeredtrademark) Dewpoint Transmitter DMT346. Incidentally, the dew point inthe present description is a value when it is assumed that anequilibrium is established between the molten salt and the atmosphere inthe vicinity of the boundary surface of the molten salt.

The aforementioned dew point can be achieved when water vapor isintroduced into the molten salt and/or the atmosphere in the vicinity ofthe boundary surface of the molten salt before the step of performingthe ion exchange and/or at the same time as the step of performing theion exchange. For example, a water vapor supply portion is added to themolten salt bath so that water vapor can be introduced into the moltensalt and/or the atmosphere in the vicinity of the boundary surface ofthe molten salt.

That is, water vapor supplied by the water vapor supply portion or gasincluding the water vapor, and water (liquid) may be introduced into themolten salt directly with bubbling, or the water vapor or the gasincluding the water vapor may be introduced into a space above themolten salt. In addition, water (liquid) itself can be dropped andintroduced onto the molten salt within an extent not causing phreaticexplosion.

When the water vapor, the gas including the water vapor or the water(liquid) (hereinafter also referred to as “water vapor etc.” simply) isintroduced, the molten salt does not have to be stirred, but it ispreferable to stir the molten salt in order to shorten the time untilreaching the equilibrium.

The time since the introduction of the water vapor etc. and untilreaching the equilibrium cannot be said definitely. The time depends onthe amount of the introduced gas or liquid, the concentration of thewater vapor, the method of the introduction, etc. However, when the dewpoint of the aforementioned atmosphere is stable and constant, it can bedetermined that the equilibrium has been established.

As for the gas including the water vapor, a gas which has no influenceon the chemical strengthening treatment can be used. For example, when adry gas A such as the air, nitrogen gas, carbon dioxide gas, etc. isintroduced into heated water 24 as shown in FIG. 3, a high-humidity gasincluding water vapor (gas including water vapor) B can be formed.

As the water 24 serving as a water vapor supply source, pure water suchas ion-exchanged water is preferably used in order to suppress scalesfrom depositing on a pipe or the like. The water 24 is, for example,heated by a water bath or the like using a water tank 25. In addition,the water 24 itself may be heated, for example, by a boiler or the likeso that the water vapor can be generated.

More specific examples of the method for introducing the water vaporetc. include (1) introducing the gas B including water vapor into aspace above the inorganic salt (molten salt 26) from the water vaporsupply portion, (2) introducing the gas B including water vapor into theinorganic salt (molten salt 26) from a bubbling portion, (3) introducingwater (liquid) directly to the inorganic salt (molten salt 26), etc.Among them, it is preferable that the atmosphere is formed by the method(1) or (2).

As a form for introducing the gas B including water vapor into the spaceabove the inorganic salt (molten salt 26), there is, for example, amethod in which the water vapor etc. supplied from the water vaporsupply portion is sprayed to above the inorganic salt or to the vicinityof the boundary surface of the inorganic salt by a spray. Introducingthe water vapor etc. by the spray is preferred because it makes it easyto control the water vapor concentration to be substantially uniform inthe space above the inorganic salt.

Incidentally, the water vapor supply portion, the bubbling portion, theintroduction portion for introducing water (liquid), or the spray is notparticularly limited, but may be provided suitably in accordance withthe apparatus. Specifically, as for the spray, either a single spray ora plurality of sprays may be provided. Particularly when the molten saltbath is large in size, the water vapor etc. may be sprayed by aplurality of sprays so as to make it easy to control the water vaporconcentration to be substantially uniform in the space above theinorganic salt.

The reason why the surface strength of the obtained chemicallystrengthened glass is enhanced by the step of ion exchange performed inthe molten salt rich in water vapor content (moisture content) can beconsidered as follows.

When carbonate ions forming the molten salt react with water, hydrogencarbonate ions and hydroxide ions are generated as shown in thefollowing formula.

CO₃ ²⁻+H₂O₄↔HCO₃ ⁻+OH⁻  [Chemical Formula 1]

Here, when the molten salt is rich in moisture content, the equilibriumin the aforementioned formula leans to the right to produce manyhydrogen carbonate ions and hydroxide ions. The hydroxide ions are ionspromoting cutting of the glass network. Accordingly, it is consideredthat formation of a low density layer which will be described later ispromoted in the glass surface due to the production of more hydroxideions

The sum of a carbonate anion concentration and a hydrogen carbonateanion concentration in the inorganic salt obtained by the followingexpression is preferably 4 mol % or higher, and more preferably 6 mol %or higher. The concentration is preferably 4 mol % or higher, becausereaction to form a low density layer in the glass surface can bepromoted.

{(carbonate anion concentration)+(hydrogen carbonate anionconcentration)} (mol %)={(carbonate anion content in inorganicsalt)+(hydrogen carbonate anion content in inorganic salt)} (mol)/{totalanion content in inorganic salt) (mol)×100

Incidentally, the carbonate anion concentration and the hydrogencarbonate anion concentration in the molten salt cannot be measureddirectly. Therefore, a part of the molten salt is taken out. Acommercially available standard solution (NaHCO₃) is diluted with purewater, and a calibration curve is created by use of a carbon dioxidemeter TiN-9004. After that, a sample solution in which the molten saltwhich has been taken out is diluted with pure water by 130 times ismeasured. On this occasion, all the hydrogen carbonate anions areconverted into carbonate anions. Therefore, the value of the carbonateanion concentration detected by the measurement corresponds to the sumof the carbonate anion concentration and the hydrogen carbonate anionconcentration.

In addition, the sum of the carbonate anion concentration and thehydrogen carbonate anion concentration is not higher than the sum of asaturated carbonate anion concentration and a saturated hydrogencarbonate anion concentration.

The low density layer is formed in, of the step of removing a part ofthe surface of the glass sheet as will be described later, a step ofbringing the glass sheet into contact with acid. The thickness of thelow density layer can be increased to 300 nm or more by ion exchange inthe atmosphere whose dew point temperature reaches 20° C. or higherowing to the introduction of water vapor, as compared with about 100 to200 nm in a conventional ion exchange step where water vapor is notintroduced.

In a glass sheet manufacturing process or a glass sheet processingprocess including a chemical strengthening treatment step, an averagedepth of cracks or latent scratches generated in the glass sheet surfaceis about 500 nm. Therefore, the thickness of the low density layer ismore preferably 500 nm or more, and even more preferably 600 nm or more.

The formed low density layer can be removed in, of the step of removinga part of the surface of the glass sheet, a step of bringing the glasssheet into contact with alkali as will be described later. Therefore, ifthe depth of all the cracks and the latent scratches in the glass sheetsurface are shallower than the thickness of the low density layer, allthe cracks and the latent scratches can be removed in the step ofbringing the glass sheet into contact with alkali.

When the cracks and the latent scratches in the glass sheet surfacecausing deterioration of strength in the chemically strengthened glassare removed, the surface strength of the chemically strengthened glasscan be further enhanced.

It is preferable that a step of washing the glass sheet is furtherincluded between the step of performing the ion exchange and the step ofremoving a part of the surface of the glass sheet. In the washing step,the glass is washed with industrial water, ion-exchanged water, etc.When industrial water is used, it is treated in accordance withnecessity. Among them, ion-exchanged water is preferred.

The washing conditions depend on a washing solution to be used therefor.When ion-exchanged water is used, it is preferable to perform washing at0 to 100° C. in order to remove the adhering salt perfectly.

In the washing step, various methods can be used. Examples of themethods include a method in which the chemically strengthened glass isimmersed in a water tank where the ion-exchanged water or the like hasbeen put, a method in which the glass sheet surface is exposed torunning water, a method in which a washing solution is sprayed towardthe glass sheet surface by a shower, etc.

((d) Step of Removing a Part of the Ion-Exchanged Surface of the GlassSheet)

The ion-exchanged glass sheet is subjected to the step of removing apart of the surface of the glass sheet ((c) to (d) of FIG. 2). It ispreferable that the step of removing a part of the surface of the glasssheet includes a step of bringing the glass sheet into contact withacid. It is more preferable that the step of removing a part of thesurface of the glass sheet further includes a step of bringing the glasssheet into contact with alkali after the step of bringing the glasssheet into contact with acid.

(Step of Bringing the Glass Sheet into Contact with Acid)

In the method for manufacturing the chemically strengthened glassaccording to the present invention, it is preferable that the step forbringing the glass sheet into contact with acid (acid treatment step) isperformed after the ion exchange step or the washing step, as the stepof removing a part of the surface of the glass sheet.

An acid treatment on the glass sheet is performed by immersing thechemically strengthened glass into an acidic solution so that Na and/orK in the chemically strengthened glass surface can be replaced by H.That is, the glass sheet surface can further include a low density layerin which a surface layer of the compressive stress layer is modified,specifically, reduced in density.

The solution is not particularly limited as long as it is acidic. Aslong as the pH is lower than 7.0, acid used as the solution is eitherweakly acidic or strongly acidic. Specifically, preferred examples ofthe solution include acids such as hydrochloric acid, nitric acid,sulfuric acid, phosphoric acid, acetic acid, oxalic acid, carbonic acid,citric acid, etc. One of those acids may be used alone, or a pluralityof them may be used in combination.

The low density layer is removed by the alkali treatment which will bedescribed later. Therefore, as the low density layer is thicker, theglass surface can be removed easily. The thickness of the low densitylayer has been described previously. From the viewpoint of the removalamount of the glass sheet surface, the thickness is preferably 300 nm ormore, more preferably 500 nm or more, and even more preferably 600 nm ormore.

From the viewpoint of removability of the glass sheet surface, thedensity of the low density layer is lower than the density of the region(bulk) deeper than the compressive stress layer subjected to the ionexchange. The thickness of the low density layer can be obtained from acycle (Δθ) measured by XRR (X-ray-Reflectometry). The density of the lowdensity layer can be obtained from a critical angle (θc) measured by theXRR.

Incidentally, in a simple manner, the formation of the low density layerand the thickness of the layer can be also confirmed by sectionalobservation of the glass with a scanning electron microscope (SEM).

(Step of Bringing the Glass Sheet into Contact with Alkali)

In the method for manufacturing the chemically strengthened glassaccording to the present invention, it is preferable that a step ofbringing the glass sheet into contact with alkali (alkali treatmentstep) is further performed after the step of bringing the glass sheetinto contact with acid. It is more preferable that a step of washing theglass sheet in the same manner as the aforementioned washing step isperformed after the step of bringing the glass sheet into contact withacid and before the step of bringing the glass sheet into contact withalkali.

An alkali treatment is performed by immersing the chemicallystrengthened glass into a basic solution so that a part or all of thelow density layer formed in the step of bringing the glass sheet intocontact with acid can be removed.

The solution is not particularly limited as long as it is basic. As longas the pH exceeds 7.0, either weak base or strong base may be used.Specifically, preferred examples of the solution include bases such assodium hydroxide, potassium hydroxide, potassium carbonate, sodiumcarbonate, etc. One of those bases may be used alone, or a plurality ofthem may be used in combination.

Owing to the aforementioned alkali treatment, a part or all of the lowdensity layer intruded by H is removed. Thus, a chemically strengthenedglass whose surface strength has been improved can be obtained.Particularly in the method for manufacturing the chemically strengthenedglass according to the present invention, the thickness of the lowdensity layer can be formed to be deeper than the depth of cracks orlatent scratches existing in the glass sheet surface. It is thereforeconsidered that the cracks or the latent scratches existing in the glasssheet surface can be removed together with the low density layer so asto further contribute to the improvement of the surface strength of theglass. Incidentally, it is preferable that a step of washing the glasssheet in the same manner as described previously is also performed afterthe alkali treatment.

EXAMPLES

Examples will be illustrated below for explaining the present inventionspecifically. However, the present invention is not limited to thoseExamples.

<Evaluation Method>

Various evaluations in these Examples were performed in the followinganalysis methods.

(Removal Amount)

The removal amount thickness of the glass sheet was obtained bymeasuring weights before and after the chemical treatment describedbelow by an analytical electronic balance (HR-202 i, made by A&DCompany, Limited) and converting the weights into thickness by means ofthe expression below. At this time, the specific gravity of the glasswas taken as 2.46 (g/cm³) for the calculation.

(thickness of removal amount per side)=((weight beforetreatment)−(weight after treatment))/(glass specific gravity)/treatedarea/2

Chemical Treatment: 6.0 weight % nitric acid (Nitric Acid 1.38 (made byKanto Chemical Co., Ltd.) was diluted with ion-exchanged water) and 4.0weight % sodium hydroxide aqueous solution (48% sodium hydroxide aqueoussolution (made by Kanto Chemical Co., Ltd.) was diluted withion-exchanged water) were prepared in beakers respectively, and theirtemperatures were adjusted to 40° C. by use of a water bath. Achemically strengthened glass obtained by a chemical strengtheningtreatment was immersed in the prepared nitric acid for 120 seconds so asto be subjected to an acid treatment. After that, the glass was washedwith pure water several times, and then immersed in the prepared sodiumhydroxide aqueous solution for 120 seconds so as to be subjected to analkali treatment. After that, the glass was washed with water to therebywash out alkali from the glass sheet surface. After that, the glass wasdried by air blowing.

(Surface Stress)

The surface compressive stress value (CS in units of MPa) and the depthof the compressive stress layer (DOL in units of μm) in the glass sheetwere measured by use of a surface stress meter (FSM-6000) made byOrihara Industrial Co., Ltd.

(Hydrogen Concentration)

According to the method described in the aforementioned (Method forMeasuring Hydrogen Concentration Profile), a hydrogen concentrationprofile was measured, and Expression (I) was derived therefrom.

(Surface Strength)

The surface strength of the glass sheet was measured by a ball-on-ringstrength test. FIG. 1 shows a schematic view for explaining theball-on-ring strength test used in the present invention. A glass sheet1 which had been mounted horizontally was pressurized by use of apressure jig 2 (made of quenched steel, having a diameter of 10 mm, andmirror-finished) made of SUS304 to measure the surface strength of theglass sheet.

In FIG. 1, the glass sheet which served as a sample was placedhorizontally on a reception jig 3 (having a diameter of 30 mm, includinga contact portion having a curvature radius R of 2.5 mm and made ofquenched steel, and mirror-finished) made of SUS304. The pressure jig 2for pressurizing the glass sheet was placed above the glass sheet.

In these Examples, the central region of the obtained glass sheet waspressurized from above the glass sheet. Incidentally, test conditionswill be described below.

Descending Rate of Pressure Jig: 1.0 (mm/min)

On this occasion, a breaking load (in units of N) with which the glasssheet was broken was regarded as BoR surface strength, and an averagevalue of BoR surface strength measured 20 times was regarded as BoRaverage surface strength. When a breaking start point of the glass sheetwas 2 mm or more away from the position where the ball was pressed, themeasured BoR surface strength was excluded from data for calculating theaverage value.

(Rubber Eraser Abrasion Test)

A rubber eraser abrasion test was performed in the following method tomeasure the AFP abrasion resistance.

The chemically strengthened glass sheet surface was cleaned byultraviolet rays, and sprayed and coated with OPTOOL (registeredtrademark) DSX (made by Daikin Industries, Ltd.) so as to form an AFPfilm substantially uniformly on the glass sheet surface. The AFP filmwas formed such that the thickness thereof was about 7.0 to 10.0 nm. TheAFP film was treated and hardened under the conditions of 50° C. and 80%RH for 1 hour.

A tester (plane abrasion tester (triple type) model: PA-300A made byDaiei Kagaku Seiki Mfg. Co., Ltd.) was used, and a rubber eraser (MINOANmade by Mirae Science Co., Ltd.) was attached to an indenter with 1 cm²wide. In a state where a load of 1 kgf was applied to the rubber eraser,the surface of the AFP film formed on the glass sheet surface was rubbedwith the rubber eraser reciprocatively 4,000 times with a stroke widthof 20 mm and at a velocity of 30 mm/sec. After that, the AFP filmsurface was dry-wiped and cleaned with a cloth (DUSPER (registeredtrademark) made by Ozu Corporation). Then, water contact angles (°) weremeasured at three places on the AFP film surface. This operation wasrepeated three times, and an average water contact angle (°) wasmeasured from the total of nine water contact angles. The water contactangles (°) on the AFP film surface were measured by a method accordingto JIS R 3257 (1999).

(Surface Roughness)

The surface roughness of the glass sheet was measured by surfaceobservation under the following conditions by use of an AFM.

Measuring Range: 1 μm×0.5 μm

Apparatus: Nanoscope V+MultiMode 8 or Dimension ICON made by BrukerCorporation

Mode; ScanAsyst

Mode Probe: RTESPA (spring constant: 40 N/m)

Samples/Line: 256

Lines: 128

Scan Rate: 1 Hz

Measuring Field: 1×0.5 μm² (aiming at a place free from contamination)

<Preparation of Chemically Strengthened Glass>

The following compositions of glasses were used in Examples andComparative Examples.

Glass Sheet A (expressed by mol % on an oxide basis): 64.4% of SiO₂,10.5% of Al₂O₃, 16.0% of Na₂O, 0.6% of K₂O, 8.3% of MgO, and 0.2% ofZrO₂

Glass Sheet B (expressed by mol % on an oxide basis): 67.0% of SiO₂,13.0% of Al₂O₃, 14.0% of Na₂O, 4.0% of B₂O₃, less than 1.0% of K₂O, 2.0%of MgO, and less than 1.0% of CaO

Example 1

9,047 g of potassium nitrate, 805 g of potassium carbonate and 148 g ofsodium nitrate were added to a pot made of stainless steel (SUS), andheated to 450° C. by a mantle heater to thereby prepare a molten salthaving 6 mol % of potassium carbonate and 4,000 weight ppm of sodium.The air introduced into water heated to 89° C. was made to flow into anatmosphere in the vicinity of the boundary surface of the molten salt tothereby make the molten salt contain water vapor.

FIG. 3 shows an apparatus used for manufacturing a chemicallystrengthened glass according to the present invention. The air was usedas a dry gas A. The air was passed into water 24 heated to 89° C. by awater tank 25 so as to be humidified. Thus, a humidified gas (air) Bcontaining water vapor was formed.

Via a path heated by a ribbon heater, the gas B containing water vaporwas introduced into a space above inorganic salt (molten salt) 26 in abath for chemical strengthening treatment. Thus, the dew point in thestep of ion exchange treatment was controlled. On this occasion, thewater vapor supply rate was 3 L/min, and the dew point in the vicinityof the boundary surface of the molten salt was 40° C.

The glass sheet A measuring 50 mm by 50 mm with a thickness of 0.7 mmwas prepared and preheated to 350 to 400° C. Then the glass sheet A wasimmersed in the molten salt at 450° C. for 90 minutes so as to besubjected to an ion exchange treatment. After that, the glass sheet Awas cooled down to the vicinity of a room temperature. Thus, a chemicalstrengthening treatment was performed. The obtained chemicallystrengthened glass was washed with water, and subjected to the followingstep.

(Acid Treatment Step)

6.0 weight % of nitric acid (Nitric Acid 1.38 (made by Kanto ChemicalCo., Ltd.) was diluted with ion-exchanged water) was prepared in abeaker, and the temperature thereof was adjusted to 40° C. by use of awater bath. The chemically strengthened glass obtained by theaforementioned chemical strengthening treatment was immersed in theprepared nitric acid for 120 seconds so as to be subjected to acidtreatment. After that, the chemically strengthened glass was washed withwater, and subjected to the following step.

(Alkali Treatment Step)

4.0 weight % of sodium hydroxide aqueous solution (48% sodium hydroxideaqueous solution (made by Kanto Chemical Co., Ltd.) was diluted withion-exchanged water) was prepared in a beaker, and the temperaturethereof was adjusted to 40° C. by use of a water bath. The chemicallystrengthened glass washed with water after the step of bringing theglass sheet into contact with acid was immersed in the prepared sodiumhydroxide aqueous solution for 120 seconds so as to be subjected to analkali treatment. After that, the chemically strengthened glass waswashed with water to thereby wash out alkali from the chemicallystrengthened glass surface. After that, the chemically strengthenedglass was dried by air blowing.

The chemically strengthened glass of Example 1 was thus obtained.

Example 2

A chemically strengthened glass was manufactured in the same manner asin Example 1, except for the conditions of the chemical strengtheningtreatment (ion exchange treatment) in which the air was passed throughwater heated to 99° C. to humidify an atmosphere in the vicinity of theboundary surface of the molten salt, and the humidified gas containingwater vapor was introduced into a space above the inorganic salt (moltensalt) in the bath for chemical strengthening treatment so that the dewpoint in the vicinity of the boundary surface of the molten salt was setat 66° C.

Example 3

A chemically strengthened glass was manufactured in the same manner asin Example 1, except that the glass sheet B measuring 50 mm by 50 mmwith a thickness of 0.55 mm was used.

Example 4

A chemically strengthened glass was manufactured in the same manner asin Example 2, except that the glass sheet B measuring 50 mm by 50 mmwith a thickness of 0.55 mm was used.

Comparative Example 1 and 2

Chemically strengthened glasses were manufactured in the same manner asin Example 1, except that the glass sheet A and the glass sheet B wereused, and the chemical strengthening treatment (ion exchange treatment)was performed on the conditions in which the molten salt for thechemical strengthening treatment did not contain K₂CO₃ as shown in Table1, the acid treatment and the alkali treatment were not performed, andthe dew point was not controlled to reach 20° C. or higher.

Various evaluations were performed on the chemically strengthenedglasses obtained thus. The treatment conditions and the evaluationresults of the glasses are shown in Table 1, FIG. 4A and FIG. 4B.Incidentally, BoR average surface strength is shown as the BoR surfacestrength.

TABLE 1 Comp. Comp. Ex. 1 Ex. 2 Ex. 1 Ex. 3 Ex. 4 Ex. 2 glass materialglass A glass A glass A glass B glass B glass B sheet thickness mm 0.70.7 0.7 0.55 0.55 0.55 salt composition K₂CO₃ mol % 6 6 — 6 6 — Naweight ppm 4000 4000 2000 4000 4000 2000 chemical strengthening dewpoint ° C. 40 66 15 40 66 15 treatment temperature/time ° C./hr 450/1.5450/1.5 450/1.5 450/1.5 450/1.5 450/1.5 acid treatment and alkalitreatment yes yes no yes yes no removal amount nm/side 420 603 — 6801190 — CS/DOL CS MPa 969 962 995 806 800 830 DOL μm 27 28 27 32 32 32SIMS value a −0.371 −0.349 −0.279 −0.312 −0.434 −0.213 value b 0.3110.301 0.219 0.365 0.324 0.239 surface strength Ave. N 1019 1065 500 661504 355 F/t² 2030 2174 1019 2186 1706 1174 AFP   0 time contact angle ∘114.5 114.9 113.9 115.0 114.8 114.7 abrasion 2000 times ∘ 107.9 102.789.6 103.6 105.1 102.2 resistance 4000 times ∘ 101.2 93.6 43.1 99.0100.2 60.0 6000 times ∘ 93.2 89.3 38.4 89.9 92.1 39.4 surface roughnessRa nm 0.57 0.55 0.17 0.72 0.73 0.19

As shown in Table 1 and FIG. 4A, in Example 1 and Example 2 which werechemically strengthened glasses according to the present invention, thesurface strength was improved conspicuously, and the AFP abrasionresistance was improved conspicuously, as compared with those inComparative Example 1 which was a chemically strengthened glass obtainedby a normal chemical strengthening treatment.

As shown in Table 1 and FIG. 4B, in Example 3 and Example 4 which werechemically strengthened glasses according to the present invention, thesurface strength was improved conspicuously, and the AFP abrasionresistance was improved conspicuously, as compared with those inComparative Example 2 which was a chemically strengthened glass obtainedby a normal chemical strengthening treatment.

From those results, a chemically strengthened glass according to thepresent invention in which a hydrogen concentration profile in a glasssheet surface layer is within a specific range is a chemicallystrengthened glass capable of attaining the surface strength and the AFPabrasion resistance compatibly.

The chemically strengthened glass according to the present invention istypically obtained by a chemical strengthening treatment in which a dewpoint is controlled. In the chemically strengthened glass according tothe present invention thus obtained, the hydrogen concentration in theglass sheet surface layer is within a specific range. When a glass has ahigh hydrogen concentration, hydrogen enters an Si—O—Si bond network ofthe glass to form Si—OH bond. Thus, the Si—O—Si bond is disconnected. Asthe hydrogen concentration in the glass is higher, parts where theSi—O—Si bond is disconnected are increased. The generated parts wherethe Si—O—Si bond is disconnected can serve as bonding start points withan AFP agent. Accordingly, when the hydrogen concentration in the glasssheet surface layer is within a specific range, it is considered thatparts forming bonds with the AFP agents can be increased. Thus, it isconsidered that a plenty of the AFP agent remains in the glass sheetsurface even after an AFP abrasion resistance test, so that the contactangle can be kept high. In this manner, it is considered that thechemically strengthened glass according to the present invention has ahigh AFP abrasion resistance.

Incidentally, any glass shows a similar contact angle before the AFPabrasion resistance test (0 time). It is considered that this is becausethe AFP agent is saturated so that some pieces of the AFP agent are notbonded with the glass but are bonded with each other.

The chemically strengthened glass according to the present invention istypically obtained by the chemical strengthening treatment in which adew point is controlled. In the chemically strengthened glass accordingto the present invention obtained thus, the surface roughness (Ra) is0.30 nm or more. Accordingly, it is considered that the surfaceroughness is higher than in the chemically strengthened glass obtainedby the conventional chemical strengthening treatment, and thus contactpoints with a rubber eraser are reduced so that the AFP abrasionresistance is improved. In the chemically strengthened glass accordingto the present invention, the surface roughness (Ra) is about 0.90 nm orless. Therefore, transparency can be also kept on the appearance of theglass sheet.

In the chemically strengthened glass according to the present invention,the hydrogen concentration in the glass is within a specific range sothat hydrogen enters the Si—O—Si bond network of the glass to form Si—OHbond to thereby disconnect the Si—O—Si bond. However, the range of thehydrogen concentration is not high enough to cause conspicuousdeterioration in surface strength. Therefore, owing to the hydrogenconcentration in the glass sheet surface layer within the specificrange, the surface strength can be kept high, and the AFP abrasionresistance can be enhanced.

Although the present invention has been described in detail withreference to its specific embodiments, it is obvious for those skilledin the art that various changes and modifications can be made withoutdeparting from the spirit and scope of the present invention. Thepresent application is based on a Japanese patent application (PatentApplication No. 2017-123365) filed on Jun. 23, 2017, the entire contentsof which are incorporated herein by reference. In addition, all thereferences cited herein are incorporated entirely.

INDUSTRIAL APPLICABILITY

A chemically strengthened glass according to the present invention is achemically strengthened glass very high in surface strength andexcellent in AFP abrasion resistance. Therefore, the chemicallystrengthened glass can be, for example, used suitably as a cover glassfor use in a display device.

REFERENCE SIGNS LIST

-   1 glass sheet-   2 pressure jig-   3 reception jig-   10 low density layer-   20 compressive stress layer-   30 intermediate layer-   24 water-   25 water tank-   26 molten salt

1. A chemically strengthened glass having a sheet shape and comprising acompressive stress layer in a glass surface layer, wherein: a straightline linearly approximating a profile of a hydrogen concentration Y withrespect to a depth X from an outermost surface of a glass sheet at X=0.1to 0.6 (μm) satisfies Expression (I):Y=aX+b  (I), wherein: Y is the hydrogen concentration (mol/L, measuredas H₂O); X is the depth from the outermost surface of the glass sheet(μm); a is −0.450 to −0.300; and b is 0.250 to 0.400.
 2. The chemicallystrengthened glass according to claim 1, having a surface roughness (Ra)of 0.30 nm or more.
 3. The chemically strengthened glass according toclaim 1, having a surface compressive stress value (CS) of 600 MPa ormore.
 4. The chemically strengthened glass according to claim 1, havinga depth of the compressive stress layer (DOL) of 10 μm or more.
 5. Thechemically strengthened glass according to claim 1, having an internaltensile stress (CT) of 72 MPa or less.
 6. The chemically strengthenedglass according to claim 1, which is used as a cover glass of a displaydevice.
 7. The chemically strengthened glass according to claim 1,having a glass composition comprising sodium.
 8. The chemicallystrengthened glass according to claim 1, having a sheet thickness of 5mm or less.
 9. The chemically strengthened glass according to claim 1,having a contact angle of 65° or more after 4,000 times of an abrasionwhen an AFP abrasion resistance is measured on conditions below by arubber eraser abrasion test, Rubber Eraser Abrasion Test Conditions: achemically strengthened glass sheet surface is cleaned by ultravioletrays, and sprayed and coated with OPTOOL (registered trademark) DSX(made by Daikin Industries, Ltd.) so as to form an AFP filmsubstantially uniformly on the glass sheet surface; a rubber eraser(MINOAN made by Mirae Science Co., Ltd.) is attached to an indenter with1 cm² wide; in a state where a load of 1 kgf is applied to the rubbereraser, an AFP film surface formed on the glass sheet surface is rubbedwith the rubber eraser reciprocatively 4,000 times with a stroke widthof 20 mm and at a velocity of 30 mm/sec; then, the AFP film surface isdry-wiped and cleaned with a cloth (DUSPER (registered trademark) madeby Ozu Corporation); thereafter, water contact angles (°) are measuredat three places on the AFP film surface; this operation is repeatedthree times, and an average water contact angle (°) is measured from atotal of nine water contact angles; the water contact angles (°) on theAFP film surface are measured by a method according to JIS R 3257(1999).